The discovery in 2002 of a gold mineralised quartz-carbon-ate vein at Ubekendt Ejland, central West Greenland, yield-ing 0.6 ppm Au over 0.7 m, led to a reconnaissance samplingproject in summer 2003. Most of the accessible quartz-car-bonate veins on the south-east coast of the island (Figs 1, 2)were sampled during boat-supported field work. Massive sul-phide mineral deposits (Fe-Zn-Pb) were located in the centreof brecciated quartz-carbonate vein systems at several placesalong the south and south-east coast of the island, and goldanomalies mainly associated with the occurrence of the mas-sive sulphides were identified. Pervasive hydrothermal altera-tion of the volcanic wall rocks surrounds the quartz-carbonate

vein systems, which comprise low-temperature mineralassemblages dominated by dolomite and veined by chal-cedony and fibrous silica. Evidence of oil migration into vol-caniclastic rocks prior to the intense hydrothermal activitywas found in several places in the form of organic carbon,interpreted to be pyrobitumen, that infills pores and cavitiesin hyaloclastites.

Geological setting

Ubekendt Ejland comprises early Palaeogene volcanic and in-trusive rocks (Fig. 1; Drever & Game 1948; Larsen 1977a, b).

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Epithermal gold and massive sulphide mineralisation inoil impregnated Palaeogene volcanic rocks of UbekendtEjland,West Greenland

Stefan Bernstein and Christian Knudsen

Geological Survey of Denmark and Greenland Bulletin 4, 77–80 (2004) © GEUS, 2004

Fig. 1. Geological map of Ubekendt Ejland.

Most quartz-carbonate veins (red lines)

visited in 2003 are located from 5 km east

of Nuunngutak to 7 km west of Qeqertalik.

Modified from Larsen (1977a, b). Spot heights

(e.g. 420) are in metres.

Most of the island consists of picritic and olivine-phyric lavas,hyaloclastites and volcanic breccias, assigned to the VaigatFormation (Fig. 1). The Vaigat Formation succession dips20–30° to the west and is overlain by plagioclase-phyric lavas,pyroclastic units and an upper lava succession including alka-line basalts (Larsen 1977b). A mafic–felsic intrusive complex,that includes layered gabbros and fine-grained granite, isfound at Saqqaata Qaqqaa in the southern part of the island.

The quartz-carbonate veins are particularly abundant inthe south-eastern part of the island, but veins also occur closeto the south-west coast, and on the north-west coast atQarusuk (Fig. 1). Most veins on the south-eastern coast canbe traced for a few kilometres northwards until they are hid-den beneath Quaternary glacial deposits. One vein systemhas been traced for 6 km along strike. The highest density ofvein systems occurs along a 5 km long stretch of the coast eastof Nuunngutak.

Quartz-carbonate veins

Some vein systems appear to be developed as relatively well-defined planar structures that extend for some distance alongstrike, judging from the orange-buff outcrop coloration. Inother cases, the veins form complex anastomosing systems,where sets of veins – often four or more – follow irregular trendsand join upwards (Fig. 2). In general, the vein systems aresubvertical. It is unclear whether the vein systems were for-med during one or several events, although at several loca-tions the veins exhibit evidence of distinct stages of brecciationand mineralisation (see below). Samples were taken up toabout 5 km west of the intrusive complex at Saqqaata

Qaqqaa, but no significant changes in mineralisation assem-blages or structures were observed.

The quartz-carbonate veins on Ubekendt Ejland typicallyconsist of banded carbonates with centres filled by quartz orcarbonate. Thin (< 1 mm) cross-cutting veins filled with quartzand/or fibrous silica also occur. The banded carbonates aremedium- to fine-grained and some veins also have coarse-grained centres with carbonate crystals up to 20 mm across(Fig. 3). The outer margins of the veins are often intenselybrecciated, showing a mixture of carbonate and quartz crys-tals and fragments, including fragments of chalcedony, set ina very fine-grained matrix of ground carbonate. The matrixsometimes shows signs of recrystallisation. Carbonate is pre-dominantly dolomite, and grades into ankerite. Some quartz-carbonate veins exhibit later sulphide mineralisation (seebelow) lining late fractures and veins (Fig. 3). The late frac-turing can be extensive and has resulted in brecciation zoneswithin the quartz-carbonate veins. The latest veining andmineralisation stage resulted in the formation of chalcedonyveins that are usually 1–2 mm thick, but chalcedony com-monly occurs in cavities and vein centres within massive sul-phides. In places, the chalcedony grades into coarser-grainedquartz (0.5 mm). Vugs filled with fibrous quartz, probablyreplacing amorphous sillica, are also common.

Sulphide and gold mineralisation

In the larger quartz-carbonate veins, the sulphide mineralisedfractures and breccias may be up to 15 cm in width and 40cm long and comprise semi-massive to massive sulphides. Inbrecciated quartz-carbonate veins, pyrite and other sulphideminerals are euhedral or subhedral and fine-grained, while in

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chalcedony sulphides

Fig. 2. Quartz-carbonate vein systems cutting picritic lava flows of the

Vaigat Formation, about 3 km east of Nuunngutak (Fig. 1). Height of cliff

about 150 m. The orange-buff coloured patterns include both the quartz-

carbonate veins and their alteration halos, typically 2–10 m wide.

Fig. 3. A 20 cm thick quartz-carbonate vein in an altered dyke, 6 km west

of Qeqertalik (Fig. 1). The vein shows several stages of brecciation, with

fragments of banded dolomite overgrown with pyrite. Some cavities are

lined with chalcedony, while others are lined with coarse calcite crystals.

Pen is 15 cm long.

the massive sulphides, pyrite, pyrrhotite and sphalerite aresubhedral and coarse-grained, most often enclosing other sul-phides such as galena, arsenopyrite, millerite, pentlandite andchalcopyrite. In most vein systems, the sulphides occur asveins and in the brecciated matrix within the quartz-carbon-ate veins, but disseminated sulphides also occur in the alteredwall rocks. Fresh sulphides have only been found in coastaloutcrops, while elsewhere they are extensively altered tolimonite.

Massive sulphide bodies comprise mainly pyrrhotite orpyrite, sphalerite, galena, arsenopyrite and native silver. Goldhas not been seen in thin section, but samples of the massivesulphides contain up to 1300 ppb Au and 110 ppm Ag, with3.8 wt% Pb and 2.2 wt% Zn. The gold appears to residewithin the sulphide-mineralised vein centres.

Alteration of wall rocks

The igneous host rocks are lava flows, volcaniclastites, hyalo-clastites and basaltic dykes, all of which have suffered exten-sive hydrothermal alteration adjacent to the quartz-carbonateveins. The alteration halos may extend up to tens of metres,but more usually are in the range of 2–5 m on either side ofthe vein system. Veins that follow older thick basaltic dykesoften only show hydrothermal alteration in a narrow (1–2 m)zone adjacent to the vein, while the remaining dyke retains itsigneous mineral assemblage.

At one location, a hyaloclastite unit is cut by a vertical, 2 mthick basaltic dyke. The eastern dyke contact is cut by a par-allel quartz-carbonate vein system, and alteration extends tothe centre of the dyke. On the east side of the vein, alteration

within the hyaloclastite host rock extends for about 8–10 m.The unaltered part of the basaltic dyke contains sparse, freshclinopyroxene phenocrysts set in a groundmass of fine-grained plagioclase, clinopyroxene and Fe-Ti oxide grains.Intersticies are filled with sericite, and vesicles with chal-cedony, carbonate and illite/phengite. In the altered part ofthe dyke, plagioclase laths in the groundmass are still visible,but are invariably altered to sericite. Former clinopyroxenephenocrysts are replaced by fine-grained yellow clay minerals,and bands of fine-grained euhedral pyrite cross-cut the rock,together with thin (50 micron) veins of carbonate or chal-cedony. Chemical analyses demonstrate that the alterationhas resulted in the loss of SiO2, MgO, CaO, Na2O, Sr andBa, and in an increase in FeOtotal, K2O, Ni, Rb, Pb andvolatiles. Other elements appear unaffected by the alteration;these include Ti, Al, P, the transition metals, Zr and the rareearth elements. These changes reflect the dissolution ofigneous silicates and growth of hydrous phyllosilicates, car-bonate and pyrite. We deduce that the added volatiles mustinclude H2O, CO2 and S.

Trace of oil on Ubekendt Ejland

Evidence of oil migration into the onshore Palaeogene vol-canic rocks is common in central West Greenland (Bojesen-Koefoed et al. 1999). On the east coast of Ubekendt Ejland(Fig. 1), minor oil staining has earlier been reported at thecontact of a dyke (Christiansen et al. 1998). During fieldwork in 2003, several localities with quartz-carbonate veinsand alteration zones on the south-east coast of UbekendtEjland were found to bear evidence of oil migration into the

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Fig. 4. Upper portion of altered

hyaloclastite, some 3 m from a

quartz-carbonate vein. The black

matrix between the lithic fragments

is organic carbon, which also fills

fine cracks in the host rock. To the

right is seen a subset of thin

carbonate veins. Pencil is 15 cm

long. About 4 km east of

Nuunngutak.

80

volcanic rocks. At one location, this is manifested by thepatchy development of black staining in altered hyaloclasticrocks (Fig. 4). In thin section, organic carbon occurs asalmost opaque material enveloping lithic fragments identicalto the surrounding altered hyaloclastite (Fig. 5). Analysis ofthe rock material yielded 1.25 wt% organic carbon, 2.89wt% carbon and 0.14 wt% sulphur. These patchy zones inthe hyaloclastites are interpreted to be remnants of oil (pyro-bitumen) that migrated into the volcanic rocks and filledpore space in the hyaloclastites. In this case the migration ofoil occurred some time before the intense hydrothermal alter-ation associated with the emplacement of the quartz-carbo-nate vein systems. Prolonged exposure to elevated temperatureshas caused thermal alteration of the oil to leave pyrobitumenas the solid residue. Assuming a density of the pyrobitumenof about 1 g/cm3, the 1.25 wt% carbon corresponds to about3 vol.% organic carbon in the rock, which in turn would rep-resent 10% of the original volume of oil. An inferred 30vol.% porosity in the matrix of the hyaloclastites appearslikely, either reflecting primary porosity or attained by disso-lution of primary pore fillings prior to the invasion of oil.

Conclusion

The presence of chalcedony and fibrous silica possibly replac-ing amorphous silica suggest that the quartz-carbonate veinsformed at a shallow level in hydrothermal conduits. Thequartz-carbonate veins are similar to epithermal Au-rich vein

mineralisations described from other volcanic terrains associ-ated with organic-rich sediments, in terms of geological set-ting, vein mineralogy, alteration of wall rocks and theevidence of migration of hydrocarbons (see e.g. Sketchley &Sinclair 1991; Sherlock & Lehrman 1995; Sillitoe et al.2002). The vein mineralogy and alteration assemblage fur-ther suggest the mineralisation is a low sulphidation type(e.g. Bonham 1988). The presence of Fe-Pb-Zn sulphides inseveral veins may indicate that the sampled level of the veinsis below that of precious metal deposition, that according tothe model of Buchanan (1981) would appear above that ofbase metal sulphides. Our findings suggest there is a poten-tial for economic precious metal mineralisation on UbekendtEjland.

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Fig. 5. Thin section of sample from locality in Fig. 4 (plane-polarised

light). The dark matrix contains lithic fragments of varying size and of a

similar composition to the host rock. Note the fine veins (arrowed) of

dark carbon-rich material extending into the altered hyaloclastite.

Authors’ addressGeological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: sb@geus.dk